Formability of paper and its improvement
|Tila||Julkaistu - 4 kesäkuuta 2015|
Formability can be defined as the ability of paper to be formed into 3D shapes without defects in appearance and functionality. Formability as a mechanical property represents a group of parameters which vary according to the type of forming process used. The primary objective of this thesis is to improve the formability of paper by increasing its extensibility. An additional objective is the characterization of formability as a mechanical property of paper and the development of a testing platform for the evaluation of formability.
It was found that the formability of paper in fixed blank forming processes is governed by the extensibility and tensile strength of paper. On the other hand, in sliding blank forming processes, it is dependent on the compressive properties of paper, elastic recovery, and the paper-to-metal coefficient of friction. The criteria of good formability are also different in these two cases, as fixed blank process formability is evaluated via the maximum depth of the shape, i.e. the deeper the shape, the better the formability. In the sliding blank process, formability is evaluated via the visual appearance of the shapes, i.e. the shapes with less profound compressive wrinkles and defects reflect good formability of paper. These results were established by comprehensive investigation of different forming processes and comparison of the outcome with the mechanical properties of paper.
Taking into account the hypothesis that the formability of paper is governed by the extensibility of paper, a set of methods for its improvement was suggested. These methods included combined high- and low-consistency treatment of fibres, spraying of agar and gelatine, in-plane compaction of paper and unrestrained drying. High-consistency treatment of fibres under elevated temperature induces permanent deformations to fibres such as microcompressions and dislocations, which in turn may decrease the axial stiffness of fibres, promoting shrinkage of paper and fibres. The low-consistency treatment straightens the fibres and induces the fibrillation of fibres to promote bonding, while microcompressions in fibres still exist. The spraying of agar and gelatine is likely to modify the character of the fibre joints by making them more deformable, and the drying shrinkage is also increased due to polymer addition. Finally, the fibre network was subjected to in-plane compaction and drying shrinkage which lead to buckling and fibre and network compression.
As a result of these treatments, the extensibility of unrestrained dried paper was increased from 4% points (untreated fibres) to 15–18% points (mechanical treatment and addition of polymers). The extensibility can be increased further by up to 30% points in one direction by compaction. This corresponds to tray-like shapes with a depth of 2–3 cm, depending on the curvature. Such values of formability are the highest reported so far in the scientific literature. The approach for the production of formable paper developed in this thesis work allows the production of a paper-based material with unmatched formability, which can replace certain types of plastic packaging. Replacement of plastics with paper improves the sustainability of packaging in general, and reduces the harmful environmental impact of non-degradable and non-renewable packaging.